<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>only-child</title>
<style>
div:only-child {
color: red;
}
</style>
</head>
<body>
<div>1</div>
<p>2</p>
<div>3</div>
<section>
<div>1</div>
<div>2</div>
<div>3</div>
<div>4</div>
<div>5</div>
</section>
<section>
<div>2</div>
</section>
</body>
</html>
匹配父元素下唯一子元素，也就是父元素下面只能包含一个div，包含其他任何一个元素就会失效
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>only-of-type</title>
<style>
div:only-of-type {
color: red;
}
</style>
</head>
<body>
<div>1</div>
<p>2</p>
<div>3</div>
<section>
<div>1</div>
<h3>2222</h3>
</section>
</body>
</html>
匹配父元素下子元素唯一，唯一和only-child不同的是，父元素下面除了一个div外可以包含其他元素，但是div必须只能一个，有二个div也会失效
